Shred the cabbage into coleslaw-sized pieces and set aside. Carefully cut up the ginger into very small pieces. Peel the potatoes and cut into very small pieces about half the size of sugar cubes. Place these in a bowl of cold water until it is time for them to be cooked.

Heat the oil in a heavy frying pan over a medium temperature. Add the black mustard seeds and chopped onion. When the mustard seeds have stopped popping and the onion appears soft, add the drained potato pieces. Sauté the potatoes for 5 minutes continually stirring so that they do not burn. Now add the shredded cabbage and keep stirring while you add the rest of the spices. Cook uncovered for 25 minutes, stirring the mixture every few minutes. The cabbage and potatoes will both turn golden brown when thoroughly cooked. When you feel that the vegetables are done, add the chopped tomato and the lemon juice. Stir once and it is ready to serve. This dish, however, does not have to be served immediately but can be kept warm in an oven that is set at a low temperature.